Two problems of current interest are the molecular basis for the neutralization of viral infectivity by antibody, and the mechanism of uncoating. Recent studies with poliovirus have suggested that antibody renders the virus noninfectious by inducing a state of refractoriness to uncoating. It has also been proposed that a single molecule of antibody is sufficient to induce this state. Several other viruses are being studied to determine if the above conclusions apply to poliovirus alone or if they are generally applicable. In related studies the effect of various treatments on the integrity of the viral protein shell is under examination. The treatments employed indicate the various kinds of chemical bonds that are involved in maintaining the highly ordered structure of the protein shell. These studies have so far suggested that hydrogen bonds, hydrophobic bonds, and bound water all contribute to the quaternary structure. Treatments that destroy infectivity by rupturing such bonds either cause an alteration of the native arrangement, or cause the release of one of the constituent polypeptides.